The thing to say is "gom bei," and while you say it you gulp down a small glass of a transparent but potent fluid called mao tai. The gom beis were popping like champagne corks, and the mao tai flowed like Coca-Cola (which also flowed) yesterday at the Liaison Office of the People's Republic of China when Vice Premier Teng Hsiao-ping gave a reception to say farewell to Washington.

Headed by Vice President Walter Mondale and Secretary of State Cyrus Vance, a large and diverse crowd of diplomats, businessmen, scholars and friends of China turned out to exchange toasts with the Chinese leader before his departure on a three-city tour of the United States.

The reception room at the Liaison Office is enormous, but it was packed solid with visitors nibbling exotic hors d'oeuvres, samping various Chinese forms of alcohol and urging one another on with "gom bei."

Invitations were hard to get for the first large reception given here by the People's Republic since the recent opening of diplomatic relations and those who came were obviously happy to be part of what more than one of them called "a historic event."

"I didn't know you were coming." said one old China hand shaking hands warmly with another. "And I didn't know you were coming," was the reply. "I haven't dared to ask anyone whether they were invited -- because, if they weren't it would be terrible."

Numerous among the guests, though not quite so numerous as they were at the Kennedy Center Monday night, were members of the National Council for U.S.-China Trade. "How are you?" asked one businessman. "All right," answered another, "just trying to make an odd dollar." "I hope they have a yen for my machinery," said a third.

In another corner, the talk was more focused on brass tacks: "I talked to Stuart, and he said the waiver may not go through for eight months. It would be foolish to make the deal until we know when the waiver will go through."

Business attitudes toward China have been changing rapidly since the United States recognized the Peking government, but some companies have had positive feelings about Peking for a long time, according to William A. Hewitt, who helped to organize the National Council for U.S.-China Trade in 1973.

"Quite a few companies were interested from the outset and joined right away," he recalled. "Others hesitated -- perhaps because their products were inappropriate for China or perhaps for other reasons. Some of them may have been doing business with Taiwan."

He said it is still too early to expect an enormous increase in trade with China: "If American companies were able to fill immense orders immediately, it would mean that they were seriously overstocked."

But he thought that long-range prospects were excellent. "The Chinese understand that the technology they need exists. They are very intelligent and have already set their priorities in oil and steel production, and particularly in the mechanization of their agriculture, which is what most interests me." Hewitt is the chairman of Deere & Co., which produces tractors.

Ambassador-designate Leonard Woodcock said he thought a major change in American business attitudes toward China has been based on a growing recognition that the Chinese can be trusted. "Now," he said, "a lot of businessmen would rather deal with the Chinese than the Russians; they're tough bargainers but fair and honest, and when they make a commitment it is firmly made."

Joseph Charyk of Comsat said that the Chinese have been using Intelsat facilities but are now developing their own satellite system for internal communications, with the United States providing the satellites.

"Satellite communication is a very logical system for a country with the size and terrain of China" he said "They will be able to leapfrog a whole generation in their communications technology."

Among the non-Chinese present, perhaps the one with the most experience of China was Bertha Sneck, an American who has been working in Peking for nearly 30 years on the editorial staff of the English-language Peking Review. "I was home on vacation when it all happened," she said. "I have been predicting this for three years, but it still came as a surprise -- it was all so sudden. Now, I'm ready to make another prediction: The Taiwan problem will be solved in a way that Americans will find completely satisfactory."

She said that her magazine, which has tended to be heavily political and aimed at a fairly small readership, is planning to broaden its appeal, and she reported that people who can teach English are now in great demand in China because "everybody is learning English."

In contrast to many diplomatic functions in Washington, the official exchange of toasts at the reception was relatively brief. Teng said that the "friendship of the American People" had made "an unforgettable impression" and proposed a toast to that friendship "and to world peace and happiness for all mankind."

In his reply, Mondale made the only remotely critical comment of the evening: "If there is one flaw to your schedule, it is the omission of the great state of Minnesota. Where else could you so readily witness our culture, our industry, our agriculture and our space-age snow-removal technology?"

Approximately 20 percent of the audience did not laugh until Mondale's remarks were translated into Chinese. Then it was "gom bei" again, and a thousand little glasses of mao tai blossomed.

There was considerable discussion among the guests on the exact meaning of "gom bei" and the ingredients and effects of mao tai. Frank Moore of the White House staff said that "gom bei" means simply, "down the hatch," but a friend said it also means "aspirin sandwiches tomorrow." This was contradicted by a veteran of several long bouts with the drink: "Mao tai does not give you a hangover, but vodka does; it's one of China's big technological advantages over Russia."

The most precise information was provided by Stanley Young, vice president of the National Council for U.S.-China Trade, but the most colorful was provided by Moore. "Each region in China makes its own kind of mao tai, and they all try to make theirs stronger than the others. The strongest I have tasted had a tiger on the label; it was called Tiger's blood, or Tiger's something."

Young has obviously studied the subject in some detail. "Mao tai," he said, "was first developed in 1704; it is made from sorghum and millet, and it may kill you but it won't give you a hangover. As for 'gom bei,' the literal meaning is 'dry cup.'"

Besides mao tai, the bar offered a choice of two Chinese wines which were not unpleasant in taste and relatively mild in alcoholic content. A Chinese whiskey seemed to be trying to taste like Scotch (the label spelled it "whisky," in the Scottish style), and didn't quite make it. An alternative bar (which seemed to get most of the patronage of the Mao-jacketed Chinese) offered Tuborg beer, pineapple or tomato juice and a selection of soft drinks. Coca-Cola and Pepsi were both available, but the Coke seemed to be going faster.